This invention relates generally to electrical power conversion methods and circuits, and more particularly to isolated soft-switched resonant converters as are required to provide controlled and reliable startup.
A typical method to regulate a resonant converter is a frequency modulation control scheme (PFM) with a control to frequency dependency, as shown in FIG. 1. The switching frequency Fsw decreases linearly as the control signal Vctrl increases. Accordingly, the output power of the converter increases towards nominal/maximal output power. The operation of a resonant converter close to its series-resonance requires only small changes in the switching frequency because only a small change in the gain of the converter is required to compensate for load changes. This is much different if the resonant converter output voltage is regulated over a wide voltage range, as is typically required during start-up or shut-down of the converter. In such cases the converter gain needs to go to zero, which requires a theoretically impractical infinite switching frequency.
Therefore, the commonly used limited frequency control range has certain limitations during startup of the resonant converter. Typically, a resonant converter cannot regulate its output to zero or low voltages, resulting in internal/external excessive current stress, hard switching, or the inability to linearly ramp-up the output. If a linear ramp-up or ramp-down is required, additional measures are needed to enable a closed loop ramp control.
One possible solution is a pulse gating control so that when the output voltage overshoots the reference value, the control pulses for the primary switches are disabled. When the output voltage falls below the reference value, the control pulses are enabled again. This helps control the output voltage during ramp-up/down. However, because of the limited frequency range of the control circuit the power stage is working in hard-switching conditions and can potentially fail or its reliability can be compromised. This happens across load and input voltage ranges.
U.S. Pat. No. 8,018,740 describes another approach based on switching between operation modes, where a converter is controlled by pulse width modulation at a constant maximum frequency during the startup and by a variable frequency mode during normal operation as depicted in FIG. 2. In this case, the switching frequency increases as the control voltage decreases up to a certain point from where the frequency is clamped, and the duty cycle is decreased to lower the transferred energy as depicted in FIG. 3. This provides a solution for a substantially linear startup but results in: (a) hard switching operation for the primary switching devices, thereby causing power stage failures in certain circumstances; and (b) the output voltage exhibits an excessive ripple/oscillation due to a non-monotonic transfer function of the resonant converter in PWM mode.
The present invention introduces a method to control the resonant converter close to its power stage characteristic frequency (CRF) to achieve zero-voltage-switching (ZVS) across a substantial load and output voltage range, while at the same time providing a closed loop control for output voltage during various operating conditions including output voltage ramp-up, ramp-down or steady-state operating conditions.